(a) Field of Invention
This invention relates to forming fabrics and particularly to those used on twin-wire paper making machines.
(b) Description of Prior Art
There are several known types of twin-wire formers all involving the injection of a stream of pulp usually containing over 99% water, into a converging gap formed by two separate endless fabrics, or wires, as they have been called, moving in the same direction and at the same speed. The gap is arranged to converge until the fabrics run together in a dewatering section with the layer of pulp sandwiched between them. The pulp is squeezed as the sandwich is drawn over a cylindrical roll or a curved stationary shoe or a series of deflector blades set in an arcuate configuration to provide support for the inner or conveying fabric while the outer or backing fabric converges forcing water out of the pulp while the fibers of the pulp remain substantially frozen in position.
Because of their greater speed capability and requiring less space and less energy, twin-wire forming machines have been gaining in popularity in recent years. However, due to a persistent tendency to produce paper having a streaky appearance, the use of twin wire formers has been limited to the production of certain grades of paper in which these quality defects are not of critical importance.
Streakiness in paper formed on a twin wire machine is generally caused by uneven disposition of pulp fibers and this has invariably been attributed to machine conditions such as incorrect setting of machine components related to the head box or slice jet or to improper setting of shoes or deflector blades in the dewatering section of the machine.
It has now been found that a cause of streakiness is unequal thickness of the sheet-like jet stream of pulp stock that is injected into the wedge-shaped converging gap between the two fabrics before they pass over the cylindrical roll or arcuate shoe. The thickness variations tend to deflect the outer backing fabric into shallow wrinkles or gullies disposed in the running direction of the machine. A gully that separates the fabrics will tend to hold a higher concentration of fibers in that area which will result in a more opaque streak in the paper. There will be a correspondingly lower concentration of pulp fibers in the areas adjacent to the gully, which results in a less opaque streak in the paper thus exaggerating the condition.
A factor that influences uneven constitution of the jet stream of pulp is that the distance of travel of the jet from the slice outlet to the point of impingement on one or other of the fabrics is necessarily quite long, in the order of about 40 cm or more on some machines and at least 25 cm on most small machines. Before this distance from the slice outlet is reached, the ribbon-like jet has lost its smooth character on both surfaces and begins to to have a corrugated appearance of an irregular character. These corrugations, which extend in the machine direction, appear as thickness variations in the cross-machine direction, and may be caused by minor defects in the slice lips, by the adherence of pulp stock or foreign objects or even by turbulence in the head box itself. However, no matter if the slice is virtually perfect in manufacture and is maintained in perfect condition, the jet stream will invariably become irregular within a distance of about 25 cm or less. Any defects such as those mentioned above simply worsen the condition.
It has further been found that if the cross-machine stiffness of the outer fabric of certain twin wire formers is increased, so that the tendency of this fabric to form gullies is reduced, inequalities in the thickness of the jet stream of pulp will also be reduced or eliminated (ironed out) and a more uniform concentration of pulp fibers throughout will result.
Forming wires were, until fairly recently, woven with bronze warp wires and brass or bronze weft wires. The metal cloth was woven in a semi-twill single layer pattern. It was inherently stiff in the cross-machine direction and provided good pulp support because of the fineness of the mesh, making the cloth particularly well suited for use, for example, on a Bel Baie II paper forming machine. From a practical point of view it is most desirable to use forming fabrics made of woven plastic polymeric strands because of their greater flexibility and better wear and corrosion resistance. However, a disadvantage of conventional plastic fabrics, and one which limits their suitability for use on twin wire paper making machines, is that due to the natural flexibility of the plastic cross-machine strands, the fabric is prone to form into wrinkles extending in the machine direction. This condition will generally be exaggerated by the high tension under which fabrics normally are run on the twin wire forming machine as well as by thickness variations in the jet-stream as previously explained.
From the above it will be apparent that it would be advantageous to provide a plastic forming fabric having increased stiffness in the cross-machine direction. While this can be accomplished to a certain extent in conventional fabrics by increasing the size and number of cross-machine strands, this measure is not entirely satisfactory because it results in a loss in drainage capacity.